This invention relates to high-speed serial data communication, and more particularly to methods and apparatus for monitoring the width of the “eye” of a high-speed serial data signal in circuitry that is handling such a signal. The data rate of a typical high-speed serial data signal may be in the range of about 6-10 Gbps (giga-bits per second), although lower and higher data rates are also well known.
In high-speed serial interface (“HSSI”) applications, the input signal of a receiver (“RX”) integrated circuit device (“chip”) is usually attenuated and distorted due to frequency-dependent signal loss across interconnects (e.g., printed circuit board (“PCB”) traces from a transmitter (“TX”) chip on the PCB to the RX chip on the PCB). This causes inter-symbol interference (“ISI”), which affects the margins for clock and data recovery (“CDR”) circuitry on the RX chip. Various RX equalization techniques have been employed to improve the input signal before the CDR circuitry to lower the bit error rate (“BER”) of the data recovered from the serial data signal.
A common way to evaluate ISI is by examining the “eye” of the serial data signal. The eye of such a signal is effectively a super-positioning of the waveform of multiple bits in the signal on the time interval of a single bit (a so-called unit interval or UI). A diagram of a signal's eye visualizes the ISI and other jitter components of the signal. An oscilloscope can be used to display the eye of a signal if the signal is accessible to the probes of the oscilloscope.
Some users of RX chips that include equalization circuitry would like to have the ability to observe at least some aspects of the eye of a high-speed serial data signal after processing by the equalization circuitry on the chip. Such a feature can have several benefits. First, such on-chip eye monitoring capability can work like an oscilloscope to probe internal high-speed nodes of the chip that cannot be observed by probing external pins of the chip. Second, the resulting eye information shows the RX equalization results, and this can be used as a basis for making adjustments to the kinds(s) and/or amount(s) of equalization being employed. Third, on-chip eye monitoring can help a system engineer analyze, diagnose, and debug HSSI devices without probes and an oscilloscope in the field.